For 2, you could use Gemini, Thomas, Cowen, or something meaning twin.

For 3, it's common in the southern US for BillyBob III to be called Tre/Trey.

Roman family have the -gens or family name and then add the number to it-- so Julia Maior and JuliaMinor (elder and younger) or JuliaTertia (third). Some Korean families I know do this where the names they call each within the family are "older sister" or "youngest" or "middle."

I can list some more number names when I find my Oxford Latin Dictionary but I learned last year in class that in most Roman families the boys would get individual names and girls would be named after their father. A man named Iulius (no J's in Latin) would have IuliaPrima, Iulia Secunda, IuliaTertia, etc.